Plastic or polymer optical fibers have been produced in the prior art over the past several decades. However, the prior art methods of producing the optical fibers have produced fibers that are relatively inefficient with regard to transmission efficiencies, especially when compared to glass optical fibers.
For example, for long-range optical communication a single-mode glass optical fiber has been widely used, because of its high transparency and high bandwidth. In contrast, for short-range communication, recently there has been considerable interests in the development of polymer optical fibers. In short-range communications (such as local area network systems, interconnections, the termination area of fiber to the home, and domestic passive optical network concepts), many junctions and connections of two optical fibers are necessary. In a single-mode fiber, the core diameter is approximately 5-10 .mu.m, so when one connects two fibers, a slight amount of displacement, such as a few micrometers, causes a significant coupling loss. The polymer optical fiber is one of the promising possible solutions to this problem, because commercially available polymer optical fiber usually has a large diameter such as 1 mm. Therefore, low transmission loss and high bandwidth has been required for polymer optical fibers to be used as a short-distance communication media.
Most commercially available polymer optical fibers, however, have been of the step-index type. Therefore, even in short-range optical communication, the step-index polymer optical fibers will not be able to cover the whole bandwidth of the order of hundreds of megahertz that will be necessary in fast datalink or local area network systems in the near future, because the bandwidth of the step-index polymer optical fibers is only approximately 5 MHz km.
In contrast, graded-index polymer optical fiber is expected to have a much higher bandwidth than step-index polymer optical fibers, while maintaining a large diameter. Several reports of a graded-index polymer optical fiber have been made by Koike, et al. (e.g., Ishigure, T., "Graded-index polymer optical fiber for high-speed data communication" Applied Optics Vol. 33, No. 19 pgs. 4261-4266(1994)). However, the methods described in the Koike et al. papers are gel diffusion methods of producing graded index fibers and are cumbersome and expensive.
What is needed is a low cost and simple method of producing a graded index polymer optical fiber. The method should produce a low-loss and high-bandwidth graded index polymer optical fiber and should include control of the graded refractive index in the fiber. In addition, the method should be easily adaptable to current manufacturing techniques of extruding polymer optical fiber.